From: Robert Klemme <bob.news@...>
Date: 2005-05-04T02:49:38+09:00
Subject: Re: Pickaxe page 342, Ranges in Boolean Expressions --I don't understand

Gunnar wrote:
> Jacob Fugal wrote:
>
>> On 5/3/05, Gunnar <gunnar.larsson2@comhem.se> wrote:
>>> Hi all
>>> I can not make sense of the boolean ranges. The example
>>>    (11..20).collect {|i| (i%4==0.. i%3==0)?i:nil} => [nil, 12, nil,
>>> nil, nil, 16, 17, 18, nil, 20].
>>>
>>> I can figure out all numbers except the '18', which in my opinion
>>> should be a 'nil' since i%3==0 is true and would cause the state to
>>> change from SET to UNSET. Is the state machine Fig 22.1 correct?
>>
>> It appears to me that the second condition in the range UNSETs, but
>> inclusively (which makes sense for the semantics of '..'). That's
>> why, for example, 12 is printed at all... 12%4==0, so it SETs,
>> retval is set to true since it is SET, then 12%3==0 so it UNSETs.
>> But the retval is already set, and it returns true, thus printing 12.
>>
>> In the same way, when we get to 18, the state machine is already SET
>> (since 16). retval is set to true since it is SET, then 18%3==0 so it
>> unsets. But just like with 12, retval is already set, so it returns
>> true and prints 18.
>>
>> For more information see [ruby-talk:91137].
>>
>> Jacob Fugal
>>
>> [ruby-talk:91137]
>> [http://www.ruby-talk.org/cgi-bin/scat.rb/ruby/ruby-talk/91137
> OK.
> Thank you for answering so very quickly
> Gunnar

It's historic: Perl does it the same way.  It's useful because you can
easily match ranges (for example of line comments, that start with
"=begin" and end with "=end" line) etc.

>> s=<<EOS
1
2
3
4
5
6
6
EOS
=> "1\n2\n3\n4\n5\n6\n6\n"
>>
?> s.each do |item|
?>   puts /3/ =~ item .. /5/ =~ item ? "inside:"+item : "outside:"+item
>> end
outside:1
outside:2
inside:3
inside:4
inside:5
outside:6
outside:6
=> "1\n2\n3\n4\n5\n6\n6\n"
>>

Kind regards

    robert